1. Field of the Invention
The present invention relates to an electron probe microanalyzer (EPMA) and, more particularly, to improvements in a method of performing data processing on plural sets of X-ray image data (such as a processing subroutine for creating a scatter diagram) obtained from the same region of a sample.
2. Description of Related Art
In electron probe microanalysis (EPMA), a sample is irradiated with a sharply focused electron beam. The wavelength and strength of characteristic X-rays emanating from the sample are measured by a wavelength-dispersive X-ray spectrometer (WDS). Chemical elements contained in a quite small region of the sample are analyzed qualitatively or quantitatively. By fixing the WDS detection wavelength at the characteristic X-rays of a certain element and scanning the electron beam across the analyzed region of the sample in two dimensions, X-ray image data (mapping data) indicating a distribution of the certain element in the analyzed region can be obtained. The number of WDS units simultaneously installed in an instrument can be increased up to about five (5 channels) according to the need. Where five WDS units are installed, X-ray image data about five chemical elements can be obtained in one measurement.
FIG. 1 shows the configuration of a related art EPMA. The EPMA has an electron gun 1 producing an electron beam EB, a condenser lens system 2 for sharply focusing the beam EB onto a sample 3, and beam scanning coils 4 for scanning the beam EB over the sample 3 or directing the beam at an arbitrary position on the sample.
Secondary electrons produced from the sample 3 are detected by a secondary electron detector 5. A secondary electron signal processing circuit 6 receives the output from the detector 5, performs given processing on the signal (such as given amplification and A/D conversion), and sends the result to a control circuit 12. Characteristic X-rays emanating from the sample 3 are detected by a WDS (wavelength-dispersive spectrometer) 7 which is configured including a spectrometer 8 for spectrally dispersing the characteristic X-rays, an X-ray detector 9 for detecting the dispersed X-rays, a spectrometer control portion 10 for driving the spectrometer 8 to perform wavelength scanning or specifying a detected wavelength, and an X-ray signal processing circuit 11 receiving the output signal from the X-ray detector 9 indicating the detected X-rays, performing given processing (such as amplification and AID conversion) on the signal, and sending the result to the control circuit 12.
The control circuit 12 controls the spectrometer control portion 10. In addition, the control circuit stores the output signal from the X-ray signal processing circuit 11 indicating the detected characteristic X-rays into a data memory 13, sends the stored data to a data processing means 14 such that the data is analyzed or otherwise processed, and sends the results of the analysis and various kinds of image data to a display portion 15, where the results and data are displayed. The control circuit 12 and data processing means 14 are composed, for example, of a computer 16.
In the instrument constructed in this way, when an X-ray image of element A is obtained from a specified analyzed region, the control circuit 12 drives the spectrometer 8 via the spectrometer control portion 10 such that the characteristic X-rays of the element A to be analyzed impinge on the detector 9 and become detected. Under this condition, the electron beam is scanned over the specified analyzed region of the sample 11 by the beam scanning coils 4. The control circuit 12 stores the output signal from the X-ray detector 9 that indicates the detected X-rays into the data memory 13 such that the signal is correlated with the scan position. As a result, two-dimensional X-ray image data indicating a distribution of the element A in the specified analyzed region is obtained in the data memory 13.
Where X-ray images of two elements A and B need to be acquired from the same specified region, the spectrometer 8 is so set up that the characteristic X-rays of the element B impinge on the detector 9 and become detected. A sequence of operations for obtaining X-ray image data is performed once more.
In the foregoing description, a single WDS unit is mounted. Where the EPMA is equipped with five WDS units as described previously, if the five WDS units are all used, X-ray image data about the five elements can be obtained at a time. If two sequences of operations are performed, X-ray image data about 10 elements at maximum can be derived.
Phase analysis is available as one analysis technique based on X-ray image data about plural elements obtained from the same analyzed region in this way. Using X-ray image data about the elements, the correlation and compositional ratios between the elements can be examined. FIG. 5 shows one example of scatter diagram created using X-ray image data about iron (Fe) and silicon (Si). The scatter diagram is created by obtaining X-ray image data about each of the two elements consisting, for example, of 256×256 pixels and plotting the data (256×256 pixels) indicating X-ray intensity or concentration of Si versus the X-ray intensity or concentration of Fe.
Often, it takes long to obtain X-ray image data. If this is repeated as a sequence of steps, it will take longer to obtain X-ray images. If a sequence of measurements' is performed over a long time, the analyzed region might slightly shift between the first and second sequences of steps due to temperature variations or sample stage drifts.
Where the scatter diagram is created from X-ray image data about two elements, the essential premise behind this technique is that the two sets of X-ray image data have been derived from the same analyzed region. Therefore, if the regions giving rise to the sets of X-ray image data about the two elements deviate, the reliability of the scatter diagram deteriorates greatly.
In a case where X-ray image data about two elements are obtained by the same sequence of steps, it follows that data have been derived from the same region by simultaneous measurements. Consequently, the resulting scatter diagram has no problem. However, in a case where two sets of data are obtained by different sequences of steps and deviation between the regions cannot be neglected, the reliability of the scatter diagram will decrease greatly. This problem has become more conspicuous as the resolution of EPMA has been enhanced in recent years and X-ray image data have been obtained at higher resolution on increasing occasions.